The present invention relates to adhesive formulations and aparticularly to adhesive formulations useful in the field of coated abrasives and to coated abrasives made using such adhesives.
Coated abrasive materials are usually produced in large rolls from which the desired commercial product is cut by an automated process. One of the most useful forms of coated abrasive is in the form of a belt. Because of the conventional manufacturing process, each such belt must be produced from a strip of suitable dimensions with the ends of the strip joined together to make a continuous loop. This may be done by profiling each end such that, when overlapped and joined, the belt has a substantially uniform cross-sectional thickness across the joined area. This is known as a "lap" joint. Alternatively both ends of the strip to be joined to form the belt may be hollowed on the non-abrasive-bearing surface to accomodate a joining strip that is bonded to each of the hollowed ends. This is known as a "butt" joint.
The material of the backing on which the abrasive material is carried can be a woven or non-woven fabric, a plastic film or sheet or a paper of suitably durability.
To form the joint it is conventional to prepare the ends by treating them to ensure that the adhesive penetrates and bonds firmly to the substrate. This is particularly important if the backing has been given a substantial backfill treatment. Where the backing is a fabric, the ends may be sandblasted to open up the exposed fiber ends and allow penetration of the adhesive in to the material of the backing.
The adhesive used must meet very demanding standards to ensure that the joint remains intact throughout the useful life of the belt. This is important since a sudden joint failure while the belt is in use could be very dangerous to both the machine and the operator.
Suitable adhesives for this demanding application are described for example in U.S. Pat. Nos. 3,154,897 and 3,763,604. There are however many others that have been proposed. It is known for example that a particularly suitable adhesive for joining abrasive belt ends is based on an aqueous solution of a polyesterurethane such as is described in U.S. Pat. No. 5,256,227, in conjunction with a cross-linking agent.
The adhesive formulations of the prior art are typically applied to the prepared ends and (in the case of a butt joint) also to the joint tape used to make the link between the ends. The pre-treated joint tape needs to be kept refigerated until just before use because at room temperature the adhesive will begin to crosslink and the tape will become useless often after only one to two hours. After the adhesive has been applied it is allowed to stand for a minimum of about 30 minutes at room temperature before being placed in a heated press for 8-10 seconds to form the joint. Because the adhesive begins to cure quite quickly, it cannot be pre-mixed in bulk more than one or two hours before the time of expected use. If a pre-treated tape is used, this must be kept refrigerated to ensure that crosslinking is not advanced by the time the tape is to be used.
The consequence is that the joining of belts is a bottle neck in the belt production operation and is adaptable to only a limited amount of mechanization. Most of the work in fact has to be done by hand by skilled operators.
An adhesive formulation has now been discovered that is highly effective, requires no dwell time before going into the hot press and which can be rapidly cured in minutes or less without resulting in inadequate bonding. The formulation can be used with a dry tape, (that is, a tape that has not been pre-treated). The formulation is therefore well-adapted to use in a fully automated operation not requiring on-the-spot refrigeration facilities.